A. Field of Invention
The invention relates to a sensor arrangement for receiving seismic waves that are reflected from geological formations below the seabed. A plurality of sensor nodes is placed on the seabed preferably in an orderly manner. In addition, the invention comprises a method of managing a seismic mapping system.
B. Description of the Related Art
For sensing seismic pressure waves, and shear waves in particular, the sensor nodes must, in principle, be in contact with a solid medium. In a marine offshore environment, this implies collecting this kind of data on the seabed, in contact with the subsea sediments. In this context, shear waves or converted seismic waves have other characteristics for describing formations and hydrocarbon reservoirs, than the traditional pressure waves alone. In a marine context, pressure waves are traditionally collected by using hydrophone sensor cables that are towed behind a vessel that also expels regular pressure pulses form air guns.
In a marine context, shear waves and converted shear waves are usually obtained by generating pressure waves with a pressure pulse from air guns at a surface vessel. When such pressure waves strike the seabed and the various geological interfaces in geological formations below the seabed, some of the pressure wave energy will transform into reflecting shear wave energy or converted shear waves, which are absorbed in sensor nodes that are deployed down into the seabed. Since water dos not have shearing strength, it cannot carry shear waves, as can solids.
To obtain mapping value of the geological formations, the sensor nodes must be placed in organized matrix patterns in large numbers on the seabed, so that the reflected pressure waves and converted shear waves together can contribute to an accurate image of the geological seabed formations. In this context, an image signifies a geometric three-dimensional representation, which gives information on petrophysical characteristics of geological reservoir formations and the fluid content of these.
Of known systems, there are mainly two types; so-called “OBS” or individual seabed seismometers and multiple component seabed cables. OBS-units are dropped from the surface and sink freely down onto the seabed. The geophones are placed on the inside of a glass ball, while the hydrophone is placed on the outside. A release mechanism makes the glass balls with geophones, hydrophone, collection electronics and collected data float up to the surface, where they are collected. An iron frame platform is left on the seabed. OBS is used for rough geological mapping base don seismic refraction.
Multiple component seabed cables normally comprise three-component geophones with gimbal suspension and hydrophones lying inside and along a cable, or in windings of a cable. The cables are laid or rolled down on the seabed by dynamically positioned vessels on the surface, but in some cases they can be pulled along the seabed. The collection of pressure and shear wave reflection data occurs with the same method as with surface cables.